1. Field of the Invention
The present invention relates to a multi-stage process for the continuous phosgene-free preparation of (cyclo)aliphatic diisocyanates.
2. Discussion of the Background
Diisocyanates are useful chemical compounds which allow controlled polyaddition, of polymers such as polyurethane and polyureas, which are widely employed industrially in and uses such as foams, elastomers, thermoplastics, fibers, coatings and adhesives. Large industrial scale production of diisocyanates has previously been accomplished by the phosgenation of the diamines, using phosgene. Phosgene is notoriously difficult to manage industrially due to its corrosivity, high toxicity and high chlorine content.
A number of processes for the preparation of (cyclo)aliphatic (in the present invention, the term "(cyclo)aliphatic" corresponds to linear or branched aliphatic or cycloaliphatic) isocyanates which by-pass the use of phosgene are known. In particular these processes first convert the diamines on which the diisocyanates are based, into biscarbamates and subject the diisocyanates to thermal cleavage in a subsequent step. By avoiding the use of phosgene, these processes avoid serious environmental protection problems and an increased expenditure on safety while, at the same time, obtaining chlorine-free isocyanates. These chlorine-free isocyanates are particularly advantageous during later use in production of polyurethanes, since the absence of residual chlorine helps to avoid stress-cracking and other types of corrosion in the reaction vessels used. While these advantages are obtained in the conventional methods, each of the known methods has one or more disadvantage.
EP-PS 18,586, and EP-PS 126,300 each disclose the preparation of (cyclo)aliphatic biscarbamates in a one-pot reaction from urea, diamine and alcohol at temperatures of 160.degree. to 300.degree. C. with simultaneous removal of ammonia.
However, this simultaneous reaction of urea, diamine and alcohol provides poor reaction selectivity. The selectivity of the reaction to produce biscarbonates is reduced by side reactions which proceed unavoidably in the 160.degree.-300.degree. C. temperature range. In addition to the desired biscarbamates, considerable amounts of N-unsubstituted alkyl carbamate and dialkyl carbonate are formed by reaction of urea and alcohol, and polyureas are formed by N,N'-substitution of the urea by the diamine. These by-products must then be removed before thermal cleavage of the biscarbamates, thus requiring an additional step in the process.
EP-PS 27,953 likewise describes a one-stage process for the preparation of carbamates by reaction of urea, primary amines and alcohols, in which the yields are said to be increased by addition of N-unsubstituted carbamates and/or N-mono- or N,N'-disubstituted ureas or polyureas, which are present as intermediate products in this reaction anyway. However, this process does not produce industrially satisfactory yields.
EP-PS 28,331 discloses the preparation of biscarbamates, inter alia, by reaction of linear polyureas with alcohol in the presence of an N-unsubstituted carbamate and/or urea. This is said to prevent the formation of amines which are formed during the reaction of N,N'-disubstituted ureas with alcohol. Once again, however, the yields achieved are not industrially satisfactory.
Additionally, neither EP-PS 27,953 nor EP-PS 28,331 contains examples or indications of the reaction of bisureas with alcohol in the absence of a diamine and/or urea.
The preparation of N-monosubstituted carbamates from trisubstituted ureas and hydroxy compounds is described in DE-PS 2,258,454. U.S. Pat. No. 2,677,698 discloses a two-stage preparation monocarbamates by formation, in the first stage, of N,N'-disubstituted ureas from urea and amine without a solvent, with the carbamates being formed in the second stage upon the addition of monoalcohols.
EP-PS 126,299 discloses a continuous preparation of biscarbamates which uses a three-stage cascade of stirred tanks. The use of a cascade of stirred tanks requires, in each case, a regulated energy supply, a column for removing the ammonia formed during reaction and equipment for maintenance of pressure for each stage, and therefore a high total investment is required.
A semi-continuous preparation of biscarbamates has been described in EP-A 355,443, wherein alternately operated stirred reactors are used and work up (isolation, distillation, etc.) is carried out continuously. The discontinuous charging and emptying required in the use of a series of alternately operated stirred reactions involves undesirable additional operating and apparatus expenditure.
U.S. Pat. Nos. 2,145,242 and 2,445,518 each disclose the preparation of bisureas from urea and diamine in bulk at 130.degree. to 140.degree. C. in 3 to 4 hours with high yields. However, each method suffers from the disadvantage that, under the reaction conditions disclosed, bisureas are obtained as solids, which presents considerable process technology problems for further processing. In order to overcome this problem, it has been proposed to carry out the reaction in the presence of inert diluents or solvents, such as chlorinated benzenes, phenols and cresols (see U.S. Pat. No. 2,145,242 and JP-Sho 38-20-748), or water (see U.S. Pat. No. 2,213,578 and Bachmann et al., J. Am. Chem. Soc. 72 (1950), 3132). However, when these insert solvents or diluents are used, they must then be removed to allow further processing of the bisurea after its formation.
JP-Sho 38-20748 describes the preparation of chain-like condensation resins having urethane bonds and urea bonds. In the disclosed process, diamine and urea are reacted in the presence of diol, at temperatures of 100.degree. to 130.degree. C., in a first reaction stage to give a solid reaction product, which, in addition to minor amounts of the bisurea, contains more highly condensed products of the linear polyurea type. These highly condensed products of the linear urea type have internal urea groupings --NH--CO--N--, which remain stable during condensation to the polyurea-polyurethane at 230.degree. C. in the presence of the diol.
It is also known from EP-PS 18,586 that the reaction of urea with hexamethylenediamine in the presence of butanol at 120.degree. to 150.degree. C. gives polyhexamethylene-urea, which is then insoluble in butanol at a temperature of 190.degree. C. and cannot be converted into the biscarbamate.
Thermal cleavage of (cyclo)aliphatic biscarbamates can be carried out in the gas phase or in the liquid phase, with or without solvents and with or without catalysts. EP-PS 126,299 and 126,300 describe processes for the preparation of hexamethylenediisocyanate and isophorone diisocyanate, respectively, by cleavage of the corresponding biscarbamates in the gas phase in a tubular reactor in the presence of metallic packing at 410.degree. C. In addition to the fact that such high temperatures can be established only with expensive technology, the disclosed processes have the disadvantage that partial cracking of the reaction products takes place at this high temperature, causing deposits on the packing and blocking of the tubular reactor. Because of this partial cracking, the process and apparatus have a short service life, making the process unsuitable for industrial production.
A continuous process for the cleavage of (cyclo)aliphatic biscarbamates in the liquid phase in the presence of catalysts without solvents is described in EP-A 355,443. In this process, 1,5-diisocyanato-2-methylpentane and other (cyclo)aliphatic diisocyanates are prepared in high yield in a stirred reactor having a capacity of 200 g, with intensive boiling of the reaction mixture at 233.degree. C. under 27 mbar. A major disadvantage to this process occurs during scale-up, in that the enlargement of the cleavage reactor, necessary to increase the capacity, leads to a reduction in the ratio of heat transfer surface of the reactor to the volume of reactor contents. If the specific heating capacity of the reactor remains unchanged, the wall temperature must be increased, leading to decomposition and caking and to deterioration of the heat transfer capability. This process is therefore also unsuitable for industrial production.
Another continuous process for the cleavage of (cyclo)aliphatic biscarbamates is disclosed in EP-A 323,514, where isophorone diisocyanate is prepared in good yield from the corresponding biscarbamate in a 300 ml flask, surmounted by a column, in the presence of 100 g of partly hydrogenated terphenyl and manganese acetate as catalyst. Once again, an attempt to scale up this process causes the yield to deteriorate considerably when the capacity is increased, because of the less favorable ratio of heat transfer surface to contents, making this process also unsuitable for industrial use.
EP-PS 54,817 discloses the continuous cleavage of monocarbamates in the liquid phase without a solvent, removal of unreacted monocarbamate and isolation of the monoisocyanate cleavage products from the alcohol used in the cleavage. However, this process leads to good results only if dephlegmators are used. If a distillation column with removal of a side stream is employed, the cleavage products cannot be isolated at all or can be isolated in only a very poor yield. Because of the low separation efficiency of dephlegmators, the condensates contain the desired components of isocyanate and alcohol in only moderate purity. During further processing of the condensates, the isocyanate thus recombines with the alcohol to give the carbamate, which must be recycled again into the cleavage reaction, causing an inherent inefficiency in the process. The profitability of the process suffers as a result.
In EP-PS 61,013, the cleavage of bis- and polycarbamates in the presence of solvents and auxiliaries, such as hydrogen chloride or organic acid chlorides, is disclosed with the cleavage products being partly condensed using dephlegmators as in EP-PS 54,1317. Because of the use of solvents and auxiliaries, which are volatile under the reaction conditions and lead to contamination of the cleavage products, the profitability of the process deteriorates still further beyond that caused by the use of dephlegmators.
In EP-PS 92,738, carbamate cleavage is carried out in a thin film in a tubular reactor or thin film evaporator, in which secondary reactions are said to be suppressed by a single pass and a short residence time. Since these secondary reactions cannot be completely avoided, in spite of the presence of a catalyst and/or stabilizer, solvents are employed to prevent caking in the tubular reactor. The gaseous cleavage products are partly condensed using dephlegmators connected in series. Cleavage to give isophorone diisocyanate, as described in EP-PS 92,738, shows the disadvantages of the process. The crude isophorone diisocyanate removed from the second dephlegmator contained only 55.5% of isophorone diisocyanate. Additionally, 43.2% of monoisocyanato-monocarbamate, and 22.6% of biscarbamate were identified in the subsequently condensed crude butanol. The yield of isophorone diisocyanate was only 51.8% of the theoretical yield.
In EP-A 396,977, as in the above-mentioned EP-PS 92,738, carbamate cleavage is performed in a tubular reactor in the presence of a solvent and catalyst. The gaseous cleavage products are likewise removed by partial condensation in dephlegmators connected in series. The diisocyanate fraction--after further dilution with the solvent employed for the cleavage--is then extracted with hydrocarbons. Because the partition coefficients of isophorone diisocyanate (IPDI) and monoisocyanato-monocarbamate (IPIU) differ only slightly, however, only incomplete separation of these compounds is possible, in spite of multi-stage extraction. The process thus becomes even more uneconomical, since an additional substance is moreover introduced into the process with the extraction agent and must be recovered in a subsequent separating operation, requiring an additional investment.
EP-A 396,976 differs from EP-A 396,977 in that the cleavage is carried out either in a tubular reactor or discontinuously in a stirred reactor, the temperature and pressure being chosen such that only the alcohol is distilled off. The bottom product from the tubular reactor or the product from the stirred reactor is then worked up by extraction and distillation, so that this process is also similarly uneconomical.
Summarizing, it can be said that the cleavage of carbamates in the gas phase has a fundamental disadvantage of exposure of the reaction products to high temperatures, while cleavage in the liquid phase in the presence of solvents requires a higher energy consumption and, because of the lower space/time yield, correspondingly higher investment compared with cleavage in the liquid phase without a solvent. The dephlegmators employed for partial condensation do not operate effectively with regard to satisfactory separation of isocyanate and alcohol.
A process is needed which provides for the preparation of (cyclo)aliphatic diisocyanates which does not have the above-mentioned disadvantages.